The present invention relates to fiber optic cables and apparatus used for broadband communications, and more particularly, to providing a fiber optic transmission systems and cables which have improved lifetime attenuation performance.
Recently fiber optic transmission has become a preferred method of providing broadband communications because of its extremely low attenuation and large available bandwidth for providing advanced digital communications. In long distance transmission applications individual optical fibers are typically bundled together to form fiber optic cables; these cables usually interface with repeaters at periodic intervals so that the optical signals carried by the fibers can be restored to their desired levels after having suffered attenuation over long propagation distances. Further, the optical transmission path may include optical filters to shape and flatten composite signal response. Often the individual fibers carry multiple independent channels through the use of, for example, dense wavelength division multiplexing (DWDM) technology, with each of the channels occupying a sub-band contained within the main transmission band. The gain vs. wavelength responses of the amplifiers contained within the repeaters are typically quite sensitive to small (age-induced) changes in input signal levels. These changes in input signal levels are most often due to age-related attenuation changes in the cabled fibers. It is for this reason that the current invention is aimed at minimizing long-term, hydrogen-induced increases in attenuation that might otherwise occur.
The fiber optic cables referred to above may be used to transmit information for telephone, television and computer data in indoor and outdoor environments. These fiber optic cables are made in various configurations and designs. Some example of a few fiber optic cable designs are illustrated in U.S. Pat. Nos. 4,156,104, 5,930,431, 4,439,632, 4,477,298, 4,557,560, 4,569,704, 4,729,629 and the article by Raymond D. Tuminaro, Materials Aspects of the SL Lightguide Undersea Cable Design, MRS Bulletin, July 1988. Outside fiber optic cables may be used to span various types of geography and environments which may include terrestrial, subterranean, and submarine (e.g., undersea) applications.
Due to the cost of manufacturing and installing the fiber optic cables, it is desired that the useful life of the fiber optic cables be as long as possible, often times greater than 25 years. One of the characteristics which affects the useful life of fiber optic cables is age-dependent increases in attenuation of signals at wavelengths used for signal transmission (e.g., 1550 nm). Further, some of the characteristics which affect the usefulness of the fiber optic transmission cable systems in, for example, DWDM systems is the overall spectral response resulting from the combination of attenuation vs. wavelength response of the fibers and the gain vs. wavelength response of the amplifiers deployed along the cabled fiber transmission path. As noted above, changes in fiber attenuation give rise to changes in input levels to the amplifiers; optical amplifiers, such as those based on erbium-doped fibers, experience changes in their gain vs. wavelength responses when their input signals carried by the cabled fibers are changed. As greater bandwidth and more and more channels are carried by individual optical fibers, stability of the attenuation behavior of the fiber is an ever increasing concern. Hydrogen-induced increase in fiber attenuation over the lifetime of the system is one of the major causes for concern with respect to the composite amplifier plus fiber spectral response.
Extensive experience in the area of optical fiber cable design has shown that, while it is often possible to reduce the amount of hydrogen inside of fiber optic cables to very low levels (for example, much less than 0.01 atmosphere partial pressure), this small amount of residual hydrogen is capable of chemically reacting with the constituents of the optical fibers to a sufficient degree so as to cause a small, but measurable, increase in fiber attenuation, with an attendant degradation in the composite spectral response of the cable plus repeater iterative structure. Further, there is evidence that, as a result of some of the fiber optic cable designs and some of the environments in which fiber optic cables are installed, the amount of hydrogen in the core of optical fibers within the fiber optic cables may increase over time. Therefore, there is a need to provide an efficient and cost effective manner for reducing the fiber optic cable susceptibility to signal degradation which may be caused by hydrogen over time.
Analysis has shown that molecules of ordinary hydrogen (having individual atoms comprised of a single electron and single proton), can readily combine with the chemical constituents of optical fibers and give rise to resonant attenuation peaks, whose tails extend into the portion of the spectral band, or bands, used for the transmission of signals, thereby degrading signal transmission. Analysis has also shown that by introducing a common isotope of hydrogen, deuterium (with individual atoms comprised of an electron, a proton, and a neutron), into the process of forming individual optical fibers, the deuterium will combine with the constituents of optical fibers and give rise to resonant attenuation peaks; however, in the case of deuterium, these resonant peaks and their tails lie well outside the commonly used portion of the spectrum used for transmission, and are therefore non-degrading, or minimally degrading, to signal transmission.
The present invention is directed to a method and apparatus for improved long term signal attenuation performance of fiber optic cable and cable and/or fiber interface components. The improved long term signal attenuation performance of the fiber optic cable and apparatus is achieved by introducing an additive after the optical fibers and apparatus have been formed as part of, or precursor to, the cable and apparatus making process. For example, an additive such as deuterium, may be introduced before, during, or after the fiber optic cable or apparatus assembly process. The additive will react with and occupy available chemically active defect sites in the optical fibers and apparatus that might otherwise adversely react with ordinary hydrogen. In some preferred embodiments, deuterium may be introduced into the cabling process and/or components anywhere from the initial fiber or component storage and preparation process to the later processing stages in which optical fiber bundles are encased within the structural elements of the cable or components are deployed within an amplifier or repeater housing. The partial or complete fiber optic cable structure or apparatus housing may then act as a reaction chamber so that the additive which has been introduced during the fiber optic cable or apparatus assembly process will react with the optical fibers or components so as to occupy defect site locations in the optical fibers or fiber-interfacing components, and thereby make them unavailable for chemical combination with ordinary hydrogen. Alternatively, the deuterium may be introduced into the optical fibers just prior to forming a fiber optic cable and components or subsequent to forming the fiber optic cable and components, as long as the deuterium is made to react with available defect sites. In any case, the occupation of defect sites by deuterium reaction products improves the long term signal attenuation characteristics of the fiber optic cables and/or components because the number of defect sites that are available for chemical combination with ordinary hydrogen will have been reduced as a result of prior combinations with deuterium.
According to one exemplary embodiment of the invention deuterium is introduced into a fill material used in the fiber optic cable and/or apparatus. According to one variation of the invention, the fill material may be used as a water blocking material used to ensure liquid water or moisture does not rapidly migrate deep into the internal portions of the cables when the cable is ruptured or the cable ends are exposed to a water-rich surrounding environment, as might be the case if the fill material was not used. According to another variation of the invention, the material into which deuterium is introduced may be a material into which the optical fibers are embedded within the casing of the fiber optic cable. The deuterium may be introduced into the material by dissolving the deuterium into the material or bubbling the material with deuterium. Likewise, the deuterium may be introduced in the post-cured fiber embedding material or materials, while the material is in the post-cured or solidified state. In another exemplary embodiment of the invention, deuterium is introduced into a fiber optic apparatus housing, which contains fiber optic components, prior to sealing the housing.
According to another exemplary embodiment of the invention deuterium is introduced into the fiber optic cable by exposing the optical fibers to deuterium prior to assembling the optical fibers into a fiber optic cable; such exposure, carried out for a relatively short period of time at moderate temperature, results in deuterium molecules being diffused and dissolved into the doped silica optical fibers with or without chemically reacting with the optical fiber materials during this introductory phase. In one variation of the invention, optical fiber sets that are to be used in a fiber optic cable are immersed in a deuterium atmosphere after the optical fibers have been manufactured but prior to being embedded within a fiber optic cable. During the immersion period the deuterium diffuses into the optical fibers to create deuterium-loaded optical fibers.
In a variation of the invention, the optical fibers are placed in a pressure-sealed chamber with deuterium gas maintained at a predetermined temperature and partial pressure for a period of time. In another variation of the invention, the optical fibers are fed through a chamber including deuterium gas as the optical fibers are being fed into the fiber optic cable assembly process to be embedded in an embedding material. The optical fibers are exposed to the deuterium under processing conditions so as to diffuse a sufficient amount of deuterium into the optical fibers in anticipation of a later defect-site chemical reaction promoted by, for example, later cabling processing or storage conditions, or alternately by retaining the fibers in the deuterium immersion chamber at time and temperature conditions required to promote the desired level of defect site reactions. As noted earlier, deuterium-based chemical reaction at defect sites, which is relatively benign from a signal transmission point of view, will ensure that those same sites will not be available for the degrading reactions that might otherwise occur with ordinary hydrogen. The inclusion of this feature into cable manufacture will improve the long term stability of the signal attenuation performance of fiber optic cable.
Once deuterium is introduced at a process step, the following conditions may apply in order to suppress the chemical reactions that might otherwise occur in the presence of ordinary hydrogen. The post-deuterium introduction process conditions (temperature and time) are established such that a significant level of deuterium reaction can occur at the defect sites. As a result, deuterium reaction may occur prior to ordinary hydrogen ingress into fiber cores. Alternatively, the deuterium introduction process may be established so that the molecular density ratio of dissolved deuterium to dissolved hydrogen at the core regions of the optical fibers may be high, in order to increase the probability that some portion of active optical fiber defect sites combine with deuterium instead of ordinary hydrogen. The time of arrival of significant numbers of deuterium at the core regions of the fibers may precede, or be sufficiently coincident with, the time of arrival of significant numbers of ordinary hydrogen molecules. In the case that deuterium and hydrogen arrive at the core regions of the optical fibers at approximately the same time, the high population ratio of deuterium to ordinary hydrogen molecules will help ensure that at least some of the defect sites are occupied by deuterium reaction products.
In one variation of the invention, after deuterium is introduced into a material used in the construction of the fiber optic cable, a material which slows, reduces, or resists the rate of deuterium egress from the cable, or cable subassembly, may be used to ensure retention and diffusion of some of the deuterium into the optic fibers embedded within the fiber optic cable. In this case, during the cabling process and over time the deuterium retained within the fiber optic cable will, after diffusing into the fibers, react and combine with the defect sites in the optical fibers, lowering the probability that ordinary hydrogen can react with those same defect sites, and thereby improving the long term stability of the attenuation characteristics of the optical fibers and fiber optic cable.